This invention relates generally to a ballast for a fluorescent lamp, and more particularly to a triac dimmable compact fluorescent lamp with low power factor.
A conventional compact fluorescent lamp includes a ballast for powering the lamp. The ballast is positioned substantially within the lamp base. In order to meet constrained space requirements, it is desirable to limit the number of ballast components required for power factor correction. Without power factor correction, a compact fluorescent lamp will operate at about a power factor of 0.5. There is also a growing demand for compact fluorescent lamps which can dim, that is, which can adjust the level of light produced by the lamp. Dimming of a compact fluorescent lamp controlled by a triac dimmer is highly desirable.
A compact fluorescent lamp with a power factor of about 0.5 essentially draws current in spikes from the A.C. mains, that is, each half cycle at about the line current peak. No current is otherwise drawn from the A.C. mains during each half cycle. A triac dimmer, however, cannot effectively dim a compact fluorescent lamp which draws current only in spikes from the A.C. mains. Triac dimmers require a latching current flowing therethrough to be turned on and a holding current flowing therethrough to sustain conduction. Consequently, a triac dimmer for controlling the light output of a compact fluorescent lamp having a power factor of about 0.5 can be turned on only during the current spike and will immediately turn off after the current spike. For a triac dimmer to effectively control the light output of a compact fluorescent lamp, current should be drawn through the triac dimmer for a substantial period of time during each half cycle of the A.C. mains. Current can be drawn through the triac dimmer for a substantial period of time during each half cycle of the A.C. mains by increasing the compact fluorescent lamp power factor. Compact fluorescent lamps with high power factors, such as disclosed in U.S. Pat. No. 5,313,142, provide feedback of a high frequency signal to the input of a ballast rectifier. As the power factor is increased and especially at power factors approaching 1.0, larger sized components are required to handle increased power levels. The larger sized components are difficult to accommodate in a compact fluorescent lamp due to limited space requirements,
Accordingly, it is desirable to provide an improved triac dimmable compact fluorescent lamp. The improved triac dimmable compact fluorescent lamp should be at a low power factor thereby minimizing the need for large sized components.